Signs your roof needs cleaning

Most roof problems announce themselves long before they become expensive — if you know what to look at. You do not need to climb up there: everything in this guide is visible from the ground, an upstairs window, or with a pair of binoculars. Once a year is enough.

Moss: from a few tufts to a warning sign

A little moss in tile joints is normal, especially on older roofs. It becomes a problem when tufts join into continuous mats. Matted moss holds a permanent film of water against the tiles, which accelerates frost damage in winter and keeps the roof damp enough for more moss to grow. If you can see green spreading across whole sections of slope — particularly the north-facing side — the roof is asking for attention.

Also check where the moss is going, not just where it is. Clumps of moss in the gutters or at the base of downpipes mean growth is detaching and washing down — a sign the infestation is mature and the gutters are at risk of blocking.

Black streaks running down the slope

Those dark vertical streaks are usually an algae called gloeocapsa magma, which feeds on the limestone filler used in many tiles. It starts as faint discolouration and darkens as colonies spread with each damp season. Early on it is cosmetic; left alone it traps moisture, looks increasingly unsightly and signals that the roof surface is staying wet — the same dampness moss needs.

Streaks starting at the ridge and lengthening downwards are the classic pattern, because spores wash down the slope with rain. If this year's streaks are visibly longer than last year's, add the roof to your to-do list.

Lichen: the quiet coloniser

Lichen shows as flat, crusty patches — pale green, grey, orange or white — that look almost painted on. It grows slowly and does less damage than moss, but its root-like structures grip the tile surface firmly and can pit it over many years. More importantly, widespread lichen tells you the roof is damp and shaded, which means moss is likely on its way if it is not there already.

Treat lichen as an early warning rather than an emergency. It responds well to the same biocidal treatment used in a proper roof clean, and catching it early keeps the job simple.

What the gutters are telling you

Gutters are the roof's diary. Moss granules, tile grit and green debris collecting in them mean the roof surface is shedding — normal in small amounts, concerning in handfuls. Vegetation actually growing in the gutter means it has been blocked long enough for seeds to germinate, and water has been overflowing instead of draining.

Overflow stains on fascia boards, damp patches at the top of walls, or water pouring over the gutter edge in rain are all downstream symptoms of a roof and gutter system that needs attention. Fix the cause, not just the symptom.

What is just normal weathering?

Not everything you see needs action. Slightly uneven colouring across tiles, minor fading from sun exposure, a few lichen spots and light dust are all normal ageing — tiles weather, and a weathered roof can still be perfectly healthy. The test is change: photograph your roof once a year from the same spot and compare. Stable can wait. Spreading cannot.

One exception: cracked, slipped or missing tiles are never “just weathering”. They need repair regardless of how clean the roof is — and a clean roof makes them far easier to spot.